Autopilot/flight director underspeed protection system

Abstract

An underspeed protection system for an aircraft under autopilot control selects a target speed based upon the greater of a minimum maneuver speed and a stick shaker speed. The system then compares a monitored speed to the target speed to produce an error signal. The system also monitors vertical speed to determine if tie aircraft begins to descend. If the error signal due to the underspeed condition causes the aircraft to descend, the system provides a hold zero vertical speed signal in place of the error signal such that the aircraft seeks to maintain its altitude. The hold zero vertical speed signal overrides the underspeed error signal such that the aircraft does not pitch forward to seek an increased speed.

I claim: 
     
       1. An apparatus for producing an underspeed override signal in an aircraft having an autopilot, comprising: a flight speed monitor configured to monitor a flight speed of the aircraft and to produce an electrical signal corresponding to the monitored flight speed;   a first minimum maneuver speed selector producing an electrical signal corresponding to a first minimum maneuver speed;   a second minimum maneuver speed selector producing an electrical signal corresponding to a second minimum maneuver speed;   a target speed signal generator coupled to receive the electrical signal corresponding to the first minimum maneuver speed and the electrical signal corresponding to the second minimum maneuver speed, the target speed signal generator producing a target signal, the target signal corresponding to the greater of the first minimum maneuver speed and second minimum maneuver speed; and   an error signal generator having a first input coupled to receive the electrical signal from the flight speed monitor and a second input coupled to receive the target signal from the target speed signal generator, the error signal generator producing the override signal corresponding to the difference between the target speed and the monitored flight speed in response to the target signal and the electrical signal from the flight speed monitor, the error signal generator having an output coupled to provide the override signal to the autopilot.   
     
     
       2. The apparatus of claim 1 wherein the target speed signal generator includes: a maximum select circuit coupled to receive the electrical signals corresponding to the first and second minimum maneuver speeds, the maximum select circuit producing an initial target signal in response to the electrical signal corresponding to the greater of the first and second minimum maneuver speeds;   a latch circuit having an unlatch input, the latch circuit coupled to receive the initial target signal and operative output the initial target signal in response to an unlatch signal at the unlatch input and to output a constant signal in response to a latch signal at the unlatch input to maintain the target signal.   
     
     
       3. The apparatus of claim 2, further including: a vertical speed monitor;   a hold zero circuit coupled to the vertical speed monitor, the hold zero circuit being operative to produce a hold zero signal in response to a vertical speed less than a selected minimum vertical speed; and   a switching circuit having a first input, a second input, a control input, and a switch output, the first input being coupled to the hold zero circuit, the second input being coupled to the error signal generator, the control input being coupled to the error signal generator and the output being coupled to the autopilot, the switching circuit being responsive to a control signal at the control to couple one or the other of the first and second inputs to the switch output.   
     
     
       4. The apparatus of claim 3, further including a logic control circuit having a first output coupled to the hold zero circuit and a second output coupled to the unlatch input. 
     
     
       5. The apparatus of claim 1, wherein the hold zero speed selector includes an altitude change detector producing the electrical signal corresponding to the hold zero vertical speed. 
     
     
       6. An underspeed control system for an aircraft having a plurality of flaps, comprising: a first zero hold circuit operative to produce a first hold zero signal corresponding to a selected flight condition;   a target airspeed signal generator producing a target signal corresponding to a target airspeed;   a first underspeed signal generator, the first underspeed signal generator including: a first airspeed monitor producing a first signal corresponding to a first monitored airspeed; and   a first comparing circuit coupled to the target airspeed signal generator to receive the signal target and coupled to the first airspeed monitor to receive the first signal, the first comparing circuit being responsive to produce a first error signal indicating a difference between the target airspeed and the first monitored airspeed;   a first selection switch having a first input coupled to receive the first error signal, a second input coupled to receive the first hold zero signal, a first switching input, and a first switch output wherein the selection switch is responsive to a first switching signal at the first switching input to selectively couple the first or second input to the first switch output;   a logic control circuit having a first logic input coupled to the selection switch output and a flap signal output, the logic circuit providing a flap signal at the flap signal output in response to a signal from the first switch output; and   a plurality of flap controllers coupled to respective ones of the flaps, the controllers having control inputs coupled to the logic control circuit and responsive to control the position of the respective flaps in response to the flap signal from the flap signal output.     
     
     
       7. The underspeed control system of claim 6, further comprising: a vertical speed detector; and   a selection circuit coupled to the vertical speed detector and operative to produce the first switching signal in response to a vertical speed below a selected minimum.   
     
     
       8. The underspeed control system of claim 6, further comprising: a second underspeed signal generator, including: a second airspeed monitor producing a second signal corresponding to a second monitored airspeed; and   a second comparing circuit coupled to the target airspeed signal generator and the second airspeed monitor, the second comparing circuit producing a second error signal indicating a difference between the target airspeed and the second monitored airspeed; and   a second selection switch having a first input coupled to receive the second error signal, a second input coupled to receive the second hold zero signal, a second switching input, and a second switch output wherein the selection switch is responsive to a second switching signal at the control input to selectively couple the first or second input to the second switch output;   wherein the logic control circuit further includes a second logic input coupled to the second switch output, the logic circuit being responsive to a signal from the second switch output to provide the flap signal.     
     
     
       9. The underspeed control system of claim 8 wherein the first comparing circuit includes: a difference circuit having a first difference input coupled to the target airspeed signal generator and a second difference input coupled to the first airspeed monitor, the difference circuit further including a difference output providing a difference signal corresponding to a difference between the first signal corresponding to the first monitored speed and the signal corresponding to the target airspeed; and   a normalizing circuit having a signal input coupled to receive the difference signal, a first variable input, and a normalized signal output, the normalizing circuit producing the error signal at the normalizing output in response to the difference signal at the signal input and a first variable signal at the first variable input.   
     
     
       10. The underspeed control system of claim 9, further comprising: a flap position detector coupled to one of the flaps and having a flap output coupled to the variable input, the flap position detector being configured to produce the variable signal in response to the detected flap position.   
     
     
       11. An aircraft, comprising: a plurality of flaps;   a plurality of flap controllers coupled to the flaps and configured for control by electrical flap control signals; and   an underspeed control system having a plurality of flap control outputs coupled to the flaps, the underspeed control system including: an airspeed monitor configured to monitor an airspeed of the aircraft and to produce a monitored airspeed signal;   a minimum speed circuit providing a signal corresponding to a minimum airspeed of the aircraft;   a comparing circuit coupled to the airspeed monitor and the minimum airspeed circuit, the comparing circuit including a difference output and providing a difference signal indicative of the difference between the monitored airspeed and the minimum airspeed;   an output circuit having a first input coupled to the comparing circuit and an output coupled to the flap controllers; and   a hold zero circuit configured to Produce a selected hold zero flap control signal wherein the output circuit includes a second input coupled to the hold zero circuits the output circuit being configured to couple one of the difference signal and the hold zero signal to the output circuit, such that one of the difference signal and the hold zero signal forms the hold zero flat control signal.     
     
     
       12. The aircraft of claim 11 further comprising: a flap position detector, wherein the output circuit includes a variable gain circuit coupled to the flap position detector, the variable gain circuit being responsive to adjust the difference signal in response to the detected flap position.   
     
     
       13. An underspeed control apparatus, comprising: an airspeed monitor operative to monitor an airspeed of an aircraft and to produce an airspeed signal in response thereto;   a vertical speed monitor operative to monitor a vertical speed of the aircraft;   a target airspeed generator operative to produce a signal indicative of a minimum airspeed, said target and speed generator comprising first and second target speed generators for producing first and second target speeds;   a comparing circuit coupled to the airspeed monitor and the target airspeed generator, the comparing circuit providing an error signal in response to the monitored airspeed falling below the minimum airspeed; and   an electronic aircraft controller coupled to receive the error signal, the controller including a control circuit configured to adjust operational parameters of the aircraft in response to the error signal, the electronic controller including an override circuit coupled to the vertical speed monitor, the override circuit being operative to override the control circuit in response to the vertical speed falling below a selected minimum vertical speed.   
     
     
       14. The underspeed control apparatus of claim 13 wherein the selected minimum vertical airspeed is negative. 
     
     
       15. The underspeed control apparatus of claim 13 wherein the selected minimum vertical speed is zero. 
     
     
       16. An underspeed control circuit for an aircraft having flaps, comprising: an airspeed monitor configured to monitor an airspeed of the aircraft;   a target speed generator configured to produce a target airspeed;   an electronic comparator coupled to the airspeed monitor and the target speed generator, the comparator producing an error signal corresponding to the difference between the monitored and target airspeeds;   a flap detect circuit coupled to the flaps and configured to detect a flap position;   a boost circuit coupled to the comparator and the flap detect circuit, the boost circuit being configured to boost the error signal in response to the detected flap position to produce a boosted error signal; and   a flap controller coupled to the boost circuit and responsive to the boosted error signal to adjust the position of the flaps.   
     
     
       17. The underspeed control apparatus of claim 16 wherein the target airspeed generator includes: first and second target speed generators for producing first and second target speeds; and   a maximum select circuit operative to select the greater of the first and second target speeds as the target airspeed.